In the diagnosis and treatment of various diseases as well as in preventative health care, it is important to monitor the concentrations of certain ions (e.g. cations) in a patient's body. One cation which has merited considerable attention in the diagnosis and treatment of heart disease, kidney disease and hypertension is sodium ion.
One type of electrode useful for determining the concentration of sodium ions in a fluid is generally composed of a reference electrode of some type and an ion-selective membrane. The reference electrode is a half-cell which contributes to providing a detectable potential during an assay. The ion-selective membrane can be made of glass or a polymeric binder material and is impregnated with an ion-selective carrier and a solvent for the carrier. The ion-sensitive carrier is also known as an ionophore and is a compound which is capable of sequentially complexing the desired ion (e.g. sodium), and transporting it across the membrane interface.
A significant advance in the art is the dry operative electrode described in U.S. Pat. No. 4,214,968 (issued July 29, 1980 to Battaglia et al). The electrodes described therein have the advantage of providing reproducible potentiometric determinations of ion activity with no requirement for wet storage or preconditioning prior to use. This patent describes the electrodes as having a dried electrolyte layer comprising a solid salt dispersed in a hydrophilic binder. This electrolyte layer is also known in the art as a reference layer.
U.S. Pat. No. 4,259,164 (issued Mar. 31, 1981 to Rasch et al) describes similar dry-operative ion-selective electrodes for the determination of potassium, carbonate and the like ions. These electrodes have an electrolyte layer which can comprise buffering agents, such as phosphates, acetic acid and the like (Col. 4, lines 40-55). Similar disclosures is provided in U.S. Pat. No. 4,263,343 (issued Apr. 21, 1981 to Kim) except that the inorganic compounds sodium hydroxide and hydrochloric acid are also mentioned as buffering agents. Neither reference teaches or suggests buffering the electrolyte layer at a pH of from about 4 to about 8 with a water-soluble, organic diacid.
Dry-operative sodium ion-selective electrodes like those described in U.S. Pat. No. 4,214,968, noted above, have been commercially available for some time in EKTACHEM Clinical Chemistry slides from Eastman Kodak Company (Rochester, N.Y. U.S.A.). While being a highly reliable and important diagnostic tool in the art, it has been difficult to efficiently manufacture slides exhibiting uniform assay performance. Generally, such slides are manufactured in a wide continuous web which is cut into strips having the proper width for individual slides. Uniformity in sodium determination among individual slides randomly selected from any location in a web is required for a viable commercial product. However, it has been observed that such uniformity is not always achieved without considerable waste. Reasons for the lack of performance uniformity among individual slides of a web are unknown. Because of this problem, considerable wasted product is generated at increased economic cost.